Rates of women who are opting for preventive mastectomies, such as Angeline Jolie, have increased by an estimated 50 percent in recent years, experts say. But many doctors are puzzled because the operation doesn't carry a 100 percent guarantee, it's major surgery -- and women have other options, from a once-a-day pill to careful monitoring.

But on Tuesday, the American Red Cross reported having just over 41,000 units of blood available nationwide, less than a two-day supply. Twenty-two of America’s Blood Centers’ 94 banks — mostly in the West and South — had a day’s supply or less on hand.

Already some hospitals, particularly on the West Coast, have postponed elective surgeries because of the shortage, say the two organizations, which together supply most of the nation’s blood.

Donations low in JanuaryJanuary is traditionally a tight month for blood supplies, as donations drop in mid-December and take a while to recover. This January, a harsh flu season and bad weather in parts of the country also are blamed for keeping donations low.

To donate, people must be healthy, be at least 17 years old and weigh at l10 pounds; a health questionnaire filled out at the blood bank will assess other qualifications. Qualified donors can give blood once every 56 days.